Left in Japan while his parents move abroad, high schooler Masahiko Kisugi moves in with his widowed aunt Hitomi Mizuhara and her three daughters. Not long after, he is blackmailed into a secret affair by Hitomi, even as her daughters Nozomi and Taki pursue him too. When Hitomi discovers the tangle of hidden trysts, she settles the strife by declaring they will all share him. To keep him from flunking out, the women agree to divide up the week, portioning Masahiko out day by day.